Uncategorized —

Paleoclimatologists give a nod to peptides

A new discovery makes scientists more and less certain of past climate …

Peering into the past is always fraught with uncertainty, and historic climate reconstructions are no exception. Built from data gleaned from things like ice cores, tree rings, and sea floor sediments, these models acknowledge the fickle nature of some data and often account for it. Sediments that include the remains of calcareous marine organisms are a significant source of paleoclimate information. While calcium-rich sediments record ocean temperatures thousands of years ago, scientists have been unable to get the sorts of detail they would prefer from them. A new discovery may help paleo-oceanographers breath a little easier at night.

Scientists have long known the ratio of magnesium to calcium in marine "hard parts'—exoskeletons, shells, and the like—correlates well with ocean temperatures. More magnesium in the hard parts indicates warmer waters. By drilling into sea floor sediments, researchers can pull material from a layer of known date and test the magnesium-calcium ratio to get a rough idea of the prevailing temperatures.

The caveat, though, is the material was once made by living things, sometimes by different species of them. Those organisms may have formed their hard parts in subtly different ways, a phenomenon experts call "vital effects." To compensate for this uncertainty, climate modelers have incorporated a bit of sway into their reconstructions. That degree of uncertainty, though, may soon narrow thanks to a new study published Thursday in the journal Science.

Vital effects are "not really well understood," said Allison Stephenson, a doctoral candidate in biogeochemistry at Virginia Tech and the paper's lead author. Paleooceanographers currently apply an organism-specific coefficient depending on the source of the sediment. But the biochemistry that shapes these differences has eluded researchers. The study’s goal was to "find a root cause of the behavior," Stephenson told Ars.

Stephenson began by growing calcium crystals in different concentrations of magnesium. In one batch, she only increased the magnesium concentration. In another, she increased the magnesium levels and added a peptide chain. Peptides are the building blocks of proteins, chemical compounds found ubiquitously in all living things. Adding peptides to the solution, thought Stephenson and her colleagues, would be a simple yet elegant test: Could a living thing control how much magnesium it put into its hard parts?

To find out, Stephenson grew the crystals under an atomic force microscope, allowing her to track the crystals as they developed. She then ran the crystalline samples through a mass spectrometer to decipher the calcium-magnesium ratio. The calcite grown in peptide-enhanced solutions incorporated more magnesium than that grown without. If this weren't accounted for, it would make the waters appear warmer than they actually were

So does this throw out decades of paleoclimatology? Probably not. "I don’t think our study throws anything out the window," Stephenson said. While it may throw current models into fits of greater uncertainty, it is likely that future models will be more accurate because of this research.

"Our study is aimed towards understanding some of the causes behind the geochemical signal in biogenic minerals," Stephenson said. "There’s still a really strong temperature dependence for magnesium and calcium."